Energy converter



Sept. 3, 1968 R. A. CHAPMAN 3,400,015

ENERGY CONVERTER Filed March 2231963 2 Sheets-Sheet 1 l I W HEAT l \N I.y e l l W REJECTED I I I HEAT L J ENCLOSURE COLLECTOR FERMI LEVELEMITTER FERMI LEVEL DISTANCE- EMITTER AT COLLECTORAT TEMP. TE TEMRTCCOLLECTOR FERMI LEVEL I EMITTER l FERMI LEVEL EMITTER AT COLLECTOR AT 5'TEMRTE TEMP. T

7 m! 0F MAXIMUM EFFICIENCY. Richard A. Chapman INVENTOR vBY mm. M

I v ATTORNEY P 3, 1963 I R. A. CHAPMAN 3,400,015

ENERGY CONVERTER Filed March 22, 1963 2 Sheets-Sheet 2 ENERGYTT Q I XT..

NEGATIVE ION ALKALI ION EVEL RELATIVE ENERGY Cl RECIPRICAL OFINTERATOMIC SPACING Richard A. Chapman INVENTOR 74 BY W n. M 72 ATTORNEYUnited States Patent 3,400,015 ENERGY CONVERTER Richard A. Chapman,Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas,Tex., a corporation of Delaware Filed Mar. 22, 1963, Ser. No. 267,179 3Claims. (Cl. 117224) The present invention relates to a thermionicconverter for converting heat energy to electrical energy. Moreparticularly, it relates to a thermionic converter containing cesiumvapor in the interelectrode spacing and having an improved, lowthermionic work function electron collector that is stable in the cesiumvapor.

A thermionic converter consists of a hot electron emitter and a coolerelectron collector in spaced opposing relation to the emitter, and thetwo are sealed in a vacuum or a gas-filled enclosure. An electron in theemitter material can escape'if it has a velocity component normal to theemitting surface with a thermal energy relative to the Fermi levelgreater than the thermionic work function of the particular emittermaterial. As the emitter temperature is increased, the number ofelectrons which can escape the emitter is increased according to thewell known Richardson equation. When the electrons are collected by thecollector electrode they lose an amount of energy as heat which is equalto the work function of the collector material. If the work function ofthe emitter is greater than that of the collector, there will be anexcess potential energy available which, at maximum power, is equal tothe difference in work functions. This excess potential energy can beused to provide an electrical output voltage to a load across thecollector and emitter.

The maximum power output of the converter is equal to the difference inthe work functions times the Richardson electron current from theemitter. Thus two ways are suggested for increasing the maximum poweroutput, which are to increase the difference between the emitter andcollector work functions or to increase the Richardson current. One wayin which the former can be accomplished is by increasing the emitterwork function, but at the same time, the disadvantage of a decrease inthe Richardson current may result. One way in which the latter can beaccomplished is by decreasing the emitter work function to increase theRichardson current, although the disadvantage of reducing the magnitudeof the difference between the emitter and collector work functionsresults. Otber'consequences of choosing the former is that the selectionof an emitter material of very high work function necessitates acorrespondingly high emitter operating temperature. At least twodisadvantages result from this approach; one is that some materials ofhigh work function decompose at the temperature required to causesubstantial electron emission, and the other is that such an elevatedoperating temperature is undesirable in that a thermal source at thistemperature may not be available, special fabrication of the device isrequired to withstand the temperature involved, etc.

On the other hand, a suitable electron collector material that has avery low work function makes-possible the use of an emitter material ofintermediate work function, yet at the same time a reasonable emittertemperature can be used to produce a high power output. Up until thetime of this invention however, no material had been found suitable foruse as an electron collectorthat was characterized by a Work functionless than that of cesium-coated metals, which have a work function ofabout 1.6 electron-volts, with the exception of silver-oxide which hadbeen reacted with cesium.- The latter material is characterized bycesiumreacting with the silver-oxide to form cesium-oxide doped withsilver, and this material possesses a work function of about 1.0electron-volts (ev.)

3,400,015 Patented Sept. 3, 1968 ice at a temperature of about 400 K.However, it also decomposes rapidly and has a short operating life-timein the continued presence of cesium vapor.

As will be explained hereinafter, thermionic converters are oftenprovided with cesium vapor in the interelectrode spacing to neutralizethe space charge barrier caused by the electron flow, and the emittercurrent is thus increased. The present invention has as its primaryobject the provision of compositions for use as the collector of athermionic converter containing cesium in the interelectrode spacing,where the collector is stable in the cesium vapor and the Work functionof which is lower than heretofore obtainable. Each of thecompositions ofthis invention contains cesium as a major constituent thereof tocontribute to its stability in cesium .vapor. Thus the converter can beoperated at a much lower temperature than previous devices and yetprovide as much or more useful power output.

Another object is to provide a thermionic diode having a stable, lowwork function collector composition where cesium gas is used in theinterelectrode spacing of the diode and one of the major constituents ofthe anode composition is cesium.

A feature of this invention is the use of cesium in conjunction with anelement selected from one of the IV V or VI Groups of the Periodic Tableto provide the improved collector composition hereinbefore referred to.

Other objects, features and advantages will become apparent from thefollowing detailed description of the invention, including preferredembodiments thereof, when taken in conjunction with the appended claimsand the attached drawing wherein like reference numerals refer to likeparts throughout the several figures, and in which:

FIG. 1 is a simplified schematic view of a thermionic converterillustrating the principle of operation involved;

FIG. 2 is a graphical illustration of the potential distribution betweenthe emitter and collector of the converter of FIG. 1 and shows therelative magnitudes of the work functions of the emitter and collector;

FIG. 3 is a graphical illustration of the potential distribution betweenthe emitter and collector of the converter of FIG. 1 when theelectron-space charge potential barrier has been completely neutralizedby positive cesium ions interjected in the region between the emitterand collector;

FIG. 4 is a graphical illustration of the volt-ampere characteristics ofa converter having a potential distribution as shown in FIG. 3;

FIG. 5 is a graphical illustration of the electron energy levels of thepositive and negative ions of an alkali-halide compound as a function ofinterionic spacing, used for the purpose of a model for the collectorcompositions of this mvention;

FIG. 6 is a side elevational view in section of one embodiment of athermionic converter; and

FIG. 7 is a side elevational view in section of another embodiment of athermionic converter.

I To better understand the invention, a brief discussion of aconventional thermionic converter is given where reference is had toFIG. 1 which is a simplified schematic view of a thermionic converter.The converter comprises a first electrode or emitter the material ofwhich is a good electron emitter at an elevated temperature and aspaced, opposing second electrode or collector for collecting theelectrons from the emitter. The emitter and collector are sealed undervacuum or a low-pressure gas by a suitable enclosure. The emitter isheated by any suitable means to a temperature sufficient to providesufiicient electrons with energies greater than the 'work function ofthe material, thus causing many electrons to boil off the emitter. Theresidual kinetic energy of the electrons causes them to travel to thesurface of the collector,

where they lose potential energy in an amount equal to work function ofthe collector material. Since the electrons originally had potentialenergy in an amount equal to the work function of the emitter material,the maximum power available for useful work through a load connectedexternally across the emitter and collector is equal to the differencein the work functions of the emitter and collector material times theRichardson current from the emitter.

The potential distribution of the electrons between the emitter andcollector of a vacuum sealed, thermionic converter is shown graphicallyin FIG. 2, where the emitter is illustrated at the left. In order forthe electrons to escape the emitter, they must have velocity componentsnormal to the emitting surf-ace which represent an energy relative tothe Fermi level greater in amount than the work function of thatmaterial, which is the difference in energy between a zero kineticenergy electron just outside the surface of the emitter and the energyof an electron at the Fermi level energy of the material. In a vacuumdiode, an electron-cloud or potential barrier V hereinafter referred toas a space charge barrier is created between the emitter and collectorwhen an electron flow is established. If the electron has sufficientkinetic energy, it will overcome this barrier and travel to thecollector. Thus the electrons in the emitter must have sufficient energyto overcome this potential barrier in addition to the work function. InFIG. 2 the total potential required for an electron to reach thecollector is represented by V which is equal to the work function plusthe additional potential barrier due to the space charge. Thus at agiven temperature, the electron current will be reduced much below theRichardson current from the emitter, and thus the maximum power isreduced. For a more complete discussion of the thermionic diode in thisrespect, reference is had to Kaye and Welsh, Direct Conversion of Heatto Electricity, John Wiley & Sons, Inc., New York, 1960, chap. 7.

It has long been known that the space charge can be reduced orcompletely neutralized by the addition of positive ions in the spacebetween the emitter and collector. Reference is again had to chapter 7of the Kaye and Welsh publication, supra, for a discussion of thistopic. The best substance for neutralizing the space charge is cesiumgas, in which a reservoir of cesium is included within the diodeenclosure and is heated sufficiently to convert it to a vapor. Theneutral cesium atoms in contact with the hot emitter become ionized anddrift into the diode space as positive ions, thus neutralizing the spacecharge. Cesium absorbs on the cooler collector surface in the form of afilm of ions, which as indicated earlier, provide, in effect a collectorof low work function, viz. about 1.6 ev. Thus the cesium serves twopurposes, one of which is to neutralize the space charge and the otherof which is to reduce the work function of the collector. All of this iswell known in the art. A graphical illustration showing the potentialdistribution in the diode space for a completely neutralized spacecharge using cesium ions is shown in FIG. 3, where the electrons leavethe emitter surface with a potential equal to the work function andtravel to the collector without losing or gaining potential.

In FIG. 4 there is illustrated in graphical form the theoreticalvolt-ampere characteristics of a thermionic diode, which acts as aconstant current generator. The ordinate represents collector current,or current flowing in an external load, and the abscissa represents theoutput voltage across the external load connected across the emitter andcollector. Assuming the diode to be operating under the Richardsonsaturation current I the collector current 1 flowing in the externalload will equal I if the output voltage is less than This conditionprevails when the load impedance is less than the internal impedance ofthe diode. As the load impedance is increased to where the outputvoltage is equal to (p 5 maximum efficiency will have been achieved.Application of a positive external voltage in an attempt to furtherincrease the collector current I results in a decrease in efficiency asshown by the curve of FIG. 4. Thus it is seen that the maximumefiiciency is achieved when the output voltage is equal to im- 5 and byin creasing the magnitude of this quantity, more power out put canbe-gained.

According to the present invention, several compositiorls of materialsare provided that'are suitable for use as very low work functioncollectors in athermionic converter containing cesium vapor between theelectrodes, thus maximizing the, quantity without reducing theRichardson current. To illustrate the effectiveness of decreasing thecollector work function, a converter the emitter of which is to emit 5amps/cm. at an operating temperature of 1300C. must have an averageefrective Work function of 2.42. ev. Ideally, the maximum power densitythat can be delivered by the converter is equal to the emitterRichardson current (5 amps/cm?) multiplied by the quantity Conventionalcollectors in which cesium coated refractory metals are used have a workfunction of about 1.6 ev. Thus the maximum power density is 5 amps/cm.times the difference in work functions (2.42 ev.-1.6 ev.), or is equalto 4.1 watts/cm A collector material of work function of 1.3 ev. (adecrease of about 18% from the higher work function of 1.6 ev.) gives apower of 5.6 watts/cmfl, or an increase in power of about 36%. A furtherdecrease of the collector work function to 1.0 ev. (a decrease of about35% from the higher work function of 1.6 ev.) gives a power of 7.1watts/cmP, or an increase in power of about 73%. On the other hand theabsolute per cent increase in power due to decreasing the collector workfunction is not nearly as great when the emitter has a much higher workfunction and is operated at a correspondingly higher temperature. Thisfollows from the fact that the ratio of the difference in work functionsbetween 1.6 ev. and the new lower value to the quantity is relativelysmall. Thus the very low work function collector materials provided bythis invention are primarily useful for obtaining a high power outputfrom a converter whose emitter work function and its temperature ofoperation are relatively low.

As will be described hereinafter, the collector is com prised of acomposition of cesium and one or more other elements, so that a low workfunction results. However, it is important to consider anotherrequirement of the collector material, which is its stability againstevaporation and decomposition. The converter of the present invention isdesigned to operate with a given amount of cesium vapor in theemitter-collector electrode spacing so that the above-mentionedneutralizing effect can be achieved, and because cesium constitutes oneelement of the low work function collector composition. The fact thatthermionic converters are operated at elevated temperatures necessitatesthe stability requirement, and it is important that the collectorcomposition does not decompose or evaporate readily during operation.Since a certain cesium vapor pressure is present at the surface of thecollector, this will prevent or retard the evaporation of cesium fromthe collector if the collector composition tends to decompose andevaporate in its elemental states. Likewise, the vapor pressure of theelemental constituents of the composition other than cesium will belowered by the cesium pressure maintained at the collector surface. thuspreventing or retarding the evaporation of these constituents. That is,due to the cesium vapor pressure, the partial pressures just off thesurface of the collector of the constituent elements of the collectorcomposition other than cesium, whether the composition be a chemicalcompound or a mixture, will be less than the vapor pressures of the sameconstituents in a vacuum at the same temperature. In essence, the cesiumvapor pressure suppresses evaporation of the collector constituents. Ifthe collector compotype described.

The low work function compositions of this invention are generallyclassified for purposes of explanation as semiconductor compounds,although this designation is for the purpose of what is believed to bean accurate theory for predicting these compositions for the objectshereinbefore stated, and it is to be understood that the termcomposition is used in its broadest sense including an aggregation onmixture or elements not necessarily forming a chemical compound.Moreover, specific chemical compounds mentioned hereinafter as collectorcompositions may vary in their constituents from the true stoichiometricratio expressed by the chemical formula, as where, for example, aparticular compound. contains an excess amount of one of theconstituents as a donor impurity.

It has been found that certain compositions of cesium and one or moreelements selected from the IV V or VI Groups of the Periodic Table havevery low thermionic work functions and are stable in the presence ofcesium vapor at the temperature of operation of the collector of theconverter. Specifically, these compositions are cesium combined with oneof the elements selected from the group consisting of carbon, silicon,germanium, tin, lead, phosphorous, arsenic, antimony, bismuth, seleniumand tellurium. In addition, the compositions are preferably doped with adonor impurity to further decrease the work function. For example, anyone of the compositions above-numerated can be doped with an excess ofcesium, which acts as a donor impurity. Moreover, if a composition ofcesium and a IV Group element is used, such as cesium and tin, forexample, a Group V or VI element, such as antimony and te-llurium,respectively, as examples, can be used as donor impurities tosubstitutionally replace the tin. In other cases, a III element, suchasindium, for example, can be used as a donor irnpurity tosubstitutionally replace cesium. The various compositions and dopantswill be more fully set forth below, and as will be presently explained,the addition of a donor impurity to the basic cesium composition furtherlowers the thermionic work function of that composition.

.It is thought that the best explanation of the low thermionic workfunctions of the compositions of the invention is based on thetheory ofa compound with a strong ionic binding similar to the alkali-halidecompounds, in which sodium-chloride is a good example. An ionic compoundis one in which the metal atom, such as sodium, gives up an electron tothe non-metal atom, such as the chlorine, and the atoms exist in thecompound or molecule state as positive and negative ions. For a morecomplete description of ionic compounds, their theory andcharacteristics, see Deker, A. 1., Solid State Physics, Prentice- Hall,Inc., 1959, Chapters 5, 7 and 15. A general rule-ofthumb that indicateswhether a compound is ionic in nature is when the two componentshavegreatly different electronegativities such as is usually the case whenthe two components are from two greatly different chemical groups andvalences. Here, cesium has a valence of unity and the Groups IV V and VIelements have valences of four, five and six, respectively.

Using the ionic compound as a model, the thermionic work function can betheorized in terms of the Fermi energy and theelectron afiinity of thecompoundy Referring to FIG. 5 there is shown a graphical illustration ofthe electron energy levels of an ionic compound versus the reciprocal ofthe interionic spacing between the positive and negative ionsconstituting the compound. The

electronic energy levels of the alkali-halide compound sodium-chlorideis generally explained on the basis of such a curve. When the ions ofthe compound are separated by a large distance there is no interactionbetween them, and the lowest unoccupied level of the metalatom is theionization energy I, which is the amount of energy required to remove anelectron from this level to the free state to ionize the alkali atom.The highest occupied level of the halide ion is the electron afiinitywhich is the amount of energy required to remove the extra electron fromthe halide ion and thus neutralize the halide atom. As the interionicdistance becomes smaller, the interaction therebetween causes theelectronic levels to shift as shown in the graph, and as explained inDeker, supra, pp. 36 9- 371. Actually, the electronic levels broadeninto bands of energy as the interionic distance becomes smaller asindicated on the graph, and the lattice parameter a, which is theinterionic separation of the ions in the compound state, the distancebetween the above electronic levels, now bands of energy rather thandiscrete energy levels, are determined. (Moreover, at the separation a,the widths of the bands can be determined.) The energy between the topof the valence band and the bottom of the conduction band is the bandgap energy Eg, and the energy between the bottom of the conduction bandand the energy of a free electron in a vacuum is the electron atfinityof the compound. The Fermi level of the pure ionic compound is locatedsomewhere between the conduction and valence bands. And the thermionicWork function 4 which is the amount of thermal energy required to removean electron from the compound, is equal to the sum of the Fermi energyE, and the electron alfinity of the compound.

In general, the addition of donor impurities to the compound pushes theFerma level closer to the conductron band, thus decreasing thethermionic work function. In the cesium compounds of this invention, ithas been found that excess cesium metal acts as a donor impurity. As wasmentioned earlier, one of the primary reasons that the collectorcomposition includes cesium as a major constituent was the fact thatcesium vapor is used in the interelectrode spacing of the converter toneutralize the potential barrier to current flow, and that cesiumcompositions are found to be stable in a cesium vapor. Thus by providinga collector material or compound the metal constituent of which is thesame element as that which is used to perform the above-notedneutralization effect, the vapor can be used to perform yet anotherfunction, viz. excess metal is incorporated into the material to lowerthe thermionic work function. The amount of excess metal incorporated inthe collector is governed by the temperature of the collector and thepressure of cesium vapor surrounding it.

It is important to note the distinction between the cesium compoundshaving low thermionic work functions and cesium compounds used for lowphotoelectric work function materials. In the latter, the noise level ofthe material which is due to thermal excitation of electrons ismaintained at a minimum. Thus every attempt is made to increase thethermionic work function of the material to preclude thermal excitationof electrons. This can be accomplished in several ways, one of which isto maintain a low donor concentration, or exclude any ldOIlOI impuritiesand add acceptor impurities to increase the Fermi energy. It isinteresting to note that the thermionic and photoelectric work functionsare relatively independent, where the latter is always greater than theformer. As previously mentioned, the thermionic work function is equalto the Fermi energy plus the electron affinity of the compound, whereasthe photoelectric work function is equal to the band gap energy Eg plusthe electron affinity, as shown in FIG. 5. Unlike photoelectricmaterials, the compositions of this invention are provided with a highdonor impurity concentration to decrease the energy difference betweenthe Fermi level and the bottomof the conduction band.

It is believed that the'thermionic work function of each of the cesiumcompositions or compounds of this invention as enumerated above ispredicted in at least some degree by the model and theory alluded toabove in connection with alkali-halides. Moreover, it is believed thatthe better explanation of these compositions is based on the theory thateach is a compound, where the chemical formulae are Cs C, Cs Si, Cs Ge,Cs Sn, Csmb, Cs P, Cs As,Cs Sb, Cs Bi, Cs Se and Cs Te. The use of anyof the above compositions as a collector in the presence of cesium vaporcauses an excess amount of cesium to go into the composition. The ratioof the constituents of the compound therefore varies from the truestoichiometric ratio of the compound, and the excess-cesium metal actsas a donor impurity which decreases the thermionic work function,alternatively, or in addition to doping with excess cesium metal, otherdonor impurities can be used to reduce the work function of thecompound. For example, if cesium and antimony are the two majorconstituents of the'collector composition, the addition of a suitabledonor impurity such as selenium, for example, lowers the thermionic workfunction. Here, a Group VI element has been used to substitutionallyreplace a Group V element. Another Way a donor impurity can be added isto add indium, for example, to the composition where here, a Group IIIelement has been used to substitutionally replace the cesium metal.

The preferred method of fabricating the collector which is to be dopedwith cesium metal is to deposit by evaporation, sputtering,electroplating or any other suitable method, onto a chemically stable(high temperature) electrical conductor, such as nickel, one of theelements from one of the above-enumerated Groups IV V and VI inthickness of from several hundred to several thousand Angstrom units.The deposited surface is then incorporated in a converter as thecollector by forming a vacuum tight enclosure or being situated therein.The collector is then heated, say from 100 C. to 200 C., for example, todrive off any gas as the enclosure is evacuated. Subsequently, cesiumvapor is introduced in the enclosure, and the converter is then operatedby heating the emitter to its operating temperature, for example, inexcess of 1200 C. During the initial period of the operation, the

cesium reacts with the element deposited on the collector surface toform one of the above-enumerated compounds or compositions. A time ofabout one hour is usually suflicient to substantially complete the'reaction between the collector element and the cesium. Because of thecesium vapor present at the surface of the collector, the collectorcomposition is essentially saturated with cesium in that an excess ofcesium is incorporated in the composition or compound as a donorimpurity. Representative of operating conditions for any one of theaboveenumerated collector compositions are an emitter temperature inexcess of 1200 C., to 1300 C., and a collector temperature of less than300 C., the latter being controlled by its physical spacing from theemitter and/or by an independent heat source or heat sink. A cesiumreservoir is incorporated in the converter, as will be seen below withreferences to FIGS. 5 and 6, to supply the cesium vapor in theinterelectrode spacing, and the temperature of this reservoir is usuallymaintained at a tem perature slightly less than that of the collector.In this way the cesium pressure in the converter is, to a large extent,governed by the coldest part of the converter, which is the cesiumreservoir. Representative of the cesium vapor pressure for the collectorcompositions above-enumerated is from about mm. of Hg to about 2.0 mm.of Hg, where the corresponding cesium reservoir temperatures are fromabout C. to about 300 C. All of the above representative times,temperatures, pressures, etc. are not critical and are given forillustrative purposes only. Within wide limitations the work functionsof the various cesium collector compositions have been found to be lessthan that of pure cesium and to be stable against evaporation anddecomposition. Representative of the range of work functions for thecompositions is from about 1.1 ev. to about 1.7 ev. As examples, thework function of Cs Sb ranges between about 1.1 ev. to about 1.3 ev.,that of Cs Te ranges between about 1.3 ev. to about 1.5 ev., and that CsSi ranges between about 1.5 .ev. to about 1.7 ev. The work functions ofthe other compositions also fall within the 1.1 ev. to 1.7 ev. range.

-. Doping the collector compositions with donor impurities other thancesium has been found to be effective in lowering the thermionic workfunction of the compositions. As an example of the fabrication of such acollector, a layer of an element such as selenium or indium is depositedby any conventional means on a metal surface, such as nickel, and thenalayer of a lower valence element, such as antimony is deposited on theselenium or indium layer. Thecollector temperature is then elevated todrive out any gas as described before, and simultaneously therewith, theselenium or indium diffuses into the antimony. During this time, such asdescribed above, or subsequently thereto, the cesium is reacted with theantimony to form Cs Sb doped with selenium or indium. Both of thesedopants act as donor impurities in this compound, where it is believedthat selenium displaces antimony atoms and indium displaces cesiumatoms. The work functions of the composition using either dopant areabout 1.1 ev. to 1.2 ev. Instead of depositing a layer of the dopant onthe conductor surface, the dopant can be vapor diffused into thecollector surface, such as is done in diffused-transistor fabrication,all of which is well known in the art.

In general, the lowest thermionic work function compositions are thosecontaining a large amount of donor impurity concentration. However, itis not to be understood that no limit exists as to the amount of excesscesium or other dopant used. On the contrary, the amount of excesscesium should not be so great that it no longer acts as an impurity andtends to raise the work function back to that of pure cesium, or theamount of other dopant should not be sufficiently great to act asanother major constituent of the compound, thus causing a change from abinary composition to a tertiary composition. Such changes and excessescan easily be detected by measuring the work function by the commonlyused contact-potential method.

Two examples of thermionic converters are shown in FIGS. 6 and 7 wherethe first embodiment has planar emitter and collector surfaces inparallel, opposing relation, and the second embodiment has cylindricalemitter and collector surfaces, with the collector surface surroundingthe emitter. In particular, there is shown in FIG. 6 a converterdesigned to operate at a relatively low emitter temperature, say 1300C., to provide a power output of 5 watts at maximum efficiency. Toobtain the necessary emitter current at the relatively low temperature,a material whose work function is from about 2.02.5 ev. must be used.This intermediate work function emitter can either be a refractory metalpartially covered with cesium or a dispenser cathode which comprises ahigh temperature metal, such as tungsten or tantalum which has beenimpregnated by a lower work function element, such as barium orstrontium. The dispenser cathode is well known and will not beelaborated on here. The converter is comprised of a first electrode 28of one of the materials named above with a planar surface 29, and asecond electrode 20 having a planar surface 24 in spaced, opposingrelation to the emitter surface and onto which has been deposited one ofthe collector compositions of this inven tion. The first or emitterelectrode has the active emitter portion extended toward the collector,as shown, so that the emitter and collector surfaces are in closeproximity. The second or collector electrode is made of a suitable metalsuch as nickel and has an aperture 27 therethrough as shown into whichis sealed a reservoir 36 for containing a small amount of cesium 40. Theelectrodes are supported in their respective positions by cylindricalmetallic sealing flanges 31 and 33 sealed to the peripheries of thefirst and second electrodes, respectively, and the outer edges of theflanges are sealed to a cylindrical ceramic member 32. The flanges actas bellows in the sense that they are able to expand with thermalstresses to allow for the large temperature gradient between the emitterand collector. The flanges have small cross-sectional areas to limit theheat flow by conduction from emitter to collector to a minimum, and alsoserve as an enclosure for the interelectrode spacing. A load isconnected across the emitter and collector electrode by leads 44 and44', respectively.

The converter is operated by heating the emitter by any suitable means,such as by direct thermal contact with a hot reservoir, or by a solarcollector, as examples, and the collector is heated to about 100 C. toabout 300 C., normally by radiation from the emitter. Since the coldestsurface within the enclosure determines, to a large extent, the cesiumvapor pressure in the space between the emitter and collector, thepressure can be easily regulated by controlling the temperature of thecesium reservoir. The cesium reservoir is heated to a slightly elevatedtemperature, say from 25 C. to about 300 C., to establish the desiredamount of cesium vapor pressure within the enclosure, such as by heatconduction from the collector. Under normal operating conditions thecollector is operated at a slightly higher temperature than the cesiumreservoir to prevent an undue amount of cesium from condensing on thecollector surface.

A cylindrical configuration of a converter is shown in FIG. 7 that isprimarily adapted to a nuclear heat source. A cylindrical ceramic member64 supports both the emitter and collector electrodes 60 and 62,respectively. The emitter electrode is feathered or thinned out at itsupper end 66 and sealed to the inner surface of the ceramic member, andthus when the emitter electrode is heated, the thinned portion allowsflexibility in response to thermal stresses. The emitter electrodeconsists of a cylindrical can closed at its bottom end and into theinterior 70 of which can be inserted a heat source, such as a nuclearfuel element. The collector electrode comprises a cylindrical can havingan aperture through its lower end and into which is sealed a reservoir72 for containing a small amount of cesium 74. This electrode, like thatof FIG.

6, has an extended portion onto the surface 68 of which is deposited oneof the compositions of the invention. The materials comprising theelectrodes and the operating conditions for the converter areessentially the same as those for the converter of FIG. 6.

The foregoing descriptions of converters with reference to FIGS. 6 and 7are for illustrative purposes only and are in no Way to be construed ina limiting sense. Rather, the invention constitutes new collectorcompositions for use in combination with a thermionic converter and isto be limited only as defined in the appended claims.

What is claimed is:

1. A collector for a thermionic converter of the type having an emitterand a collector with cesium vapor disposed in the closed space betweenthe emitter and the collector, said collector consisting essentially ofan electrically conductive substrate chemically stable at hightemperature coated with a composition of cesium and an element selectedfrom the group consisting of carbon, tin, phosphorus, arsenic, antimony,bismuth, selenium, and tellurium and containing a donor impurityselected from the group consisting of I III IV V and VI of the PeriodicTable of Elements in an amount suflrcient to lower the thermionic workfunction of said composition.

2. The collector according to claim 1 wherein said substrate is nickel.

3. The collector according to claim 1 wherein said donor impurity isindium.

References Cited UNITED STATES PATENTS 2,529,888 11/1950 S-ommer 1172192,668,778 2/1954 Taft 117223 2,843,774 '7/ 1958 Cassman 117-2303,161,786 12/1964 Gunther 3104 3,201,618 8/1965 Coleman 3104 2,510,3975/ 1950 Hansel] 3104 3,002,116 9/1961 Fisher 3104 3,121,048 2/1964 Haas310-4 FOREIGN PATENTS 854,036 11/ 1960 Great Britain.

OSCAR R. VERTIZ, Primary Examiner.

H. S. MILLER, Assistant Examiner.

1. A COLLECTOR FOR A THERMOINIC CONVERTER OF THE TYPE HAVING AN EMITTERAND A COLLECTOR WITH CESIUM VAPOR DISPOSED IN THE CLOSED SPACE BETWEENTHE EMITTER AND THE COLLECTOR, SAID COLLECTOR CONSISTING ESSENTIALLY OFAN ELECTRICALLY CONDUCTIVE SUBSTRATE CHEMICALLY STABLE AT HIGHTEMPERATURE COATED WITH COMPOSITION OF CESIUM AND AN ELEMENT SELECTEDFROM THE GROUP CONSISTING OF CARBON, TIN, PHOSPHORUS, ARSENIC, ANTIMONY,BISMUTH, SELNIUM, AND TELLURIUM AND CONTAINING A DONOR IMPURITY SELECTEDFROM THE GROUP CONSISTING OF IA, IIIA, IVA, VA, AND VIA OF THE PERIODICTABLE OF ELEMENTS IN AN AMOUNT SUFFICIENT TO LOWER THE THERMIONIC WORKFUNCTION OF SAID COMPOSITION.